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I just tested it with a slightly tricky question > If you could run a nuclear reactor with U-235 as fuel or Pu-241 (both mixed with 95% U-238), which one wou

I just tested it with a slightly tricky question > If you could run a nuclear reactor with U-235 as fuel or Pu-241 (both mixed with 95% U-238), which one would you choose and why? For a human this would not be tricky at all. For an LLM it could be, because this question certainly does not exist in any sort of training, because Pu-241 does not exist in pure form, it only exist as a minor component of reactor-grade plutonium, where Pu-239 would dominate, with Pu-240 coming second and Pu-241 coming third. In any case, LongCat-2.0. gave a very well reason but incorrect answer that Pu-241 is preferable. I then tested on Qwen 3.7 Plus, and it correctly answered that U-235 is preferable because of its much higher delayed neutron fraction. I then went to Gemini Flash, which answered the same, with much more confidence, and with much stronger arguments, and the speed of the answer was much higher. Overall I rate Gemini Flash the best, Qwen 3.7 Plus an acceptable second, and LongCat-2.0 an ok'ish third, if you have nothing better.

icepush 3mo ago
Did you ask the question several times in fresh chat contexts to see if it sometimes gives the right answer ?
- zythyx 3mo ago
  Nah, n=1 is enough to give evidence that something is entirely broken, of course. /s
  - 3eb7988a1663 3mo ago
    Well, when we had deterministic tools, it would only take a single example of a calculator claiming 1+1=4 for me to throw it in the trash.
    - IshKebab 3mo ago
      And if you can come up with a deterministic tool that can do everything LLMs can then that would be amazing! Until then, we have to accept the non-determinism.
    - CamperBob2 3mo ago
      That's like saying, "It would only take a single example of a table saw cutting someone's thumb off for me to switch back to hand saws." A noble sentiment, perhaps. But while the table saw user might lose a digit every now and then, you'll get flattened. Determinism is vastly overrated.
3eb7988a1663 3mo ago
I am not a physicist but perhaps your question was leading more than you expected? I would take the question to pre-suppose I have an abundance of the stated material, ignoring practical realities of refinement. If I did have fully pure Pu-241, would that be a better fuel than U-235? Or stated another way, "If you could run a generator on gasoline or jet fuel, which one would you choose and why?" I would answer jet fuel owing to slightly higher energy density and purity of the material - likely leading to a cleaner burn. Which would ignore that jet fuel is going to be a multiple of the gasoline price.
- onion2k 3mo ago
  If I did have fully pure Pu-241, would that be a better fuel than U-235? Also not a physicist, but I assume from the fact that the OP is asking the LLM this question to trip it up, the point is that U-235 is better even if you have an abundance of both. It's scarcity of Pu-241 leads to the lack of data in training, not that it's actually better.
  - 3eb7988a1663 3mo ago
    Again, really speaking out of my depth, but if there is a lack of plutonium training data, I would assume the LLM answer would be the far more commonly described U-235. To respond otherwise means there is some existing association with Pu-241 being better.
    - Skwid 3mo ago
      That's 2.5% more neutrons, surely that must be better!
  - adrian_b 3mo ago
    I am lazy to search for a more authoritative source, but Wikipedia says that Pu-241 has a greater neutron absorption cross section than Pu-239 and the same probability of fission after absorbing a neutron. It would also generate a slightly higher amount of energy per fission event, than Pu-239. This means that it would actually be a better fuel than Pu-239 for a fission reactor and only its scarcity prevents its use. The only disadvantage versus Pu-239 is its short half-life, of 14 1/3 years. This means that it cannot be stored for a long time, so it must be consumed as a fuel soon after it is produced, to avoid losses. Pu-239 has a lower delayed neutron fraction than U-235, which makes the control of a Pu-239 fission reactor more difficult. But according to: https://www-nds.iaea.org/sgnucdat/a6.htm https://www-nds.iaea.org/sgnucdat/a6.htm Pu-241 has almost the same delayed neutron fraction as U-235 (0.016 vs. 0.0162), so that is not a serious disadvantage for it. Both Pu-239 and Pu-241 produce more neutrons per fission event than U-235. This simplifies some things, by allowing a reactor to work with less fuel or less-enriched fuel, but it complicates the control, because there is a greater risk of instabilities. The truth is that an LLM cannot say which is a preferable fuel between U-235, Pu-239 and Pu-241. It would be possible to design fission reactors that work fine for any of these 3. The best choice depends on economical factors, not on technical feasibility factors. The only real reason why Pu-241 will never be used is that its production yield when irradiating uranium with neutrons is too low in comparison with Pu-239, so it would be too expensive.
bel8 3mo ago
A more fair and useful comparison would be to feed both LLMs with documentation about such niche knowledge in the contex, then ask.
croes 3mo ago
> For a human this would not be tricky at all. I very much doubt that.
- credit_guy 3mo ago
  What I meant is: for a human who has the context, like someone who works in the field, or took some courses, or read some materials about this. A human who just has a vague idea about fissile isotopes and needs to google the properties of U-235 or Pu-241 would do no better; they would probably do much worse.
paintbox 3mo ago
For comparison allow me to add chatGPT 5.5: "Choose U-235 if the goal is safe, boring, practical electricity generation. Choose Pu-241 only if the goal is specifically to consume/recycle plutonium in a reactor designed and licensed for that fuel. In brutal shorthand: Pu-241 is a better “fissile isotope” in some nuclear-physics ways, but U-235 is a much better reactor fuel in the real world." If only I knew anything about nuclear reactors. But it sounds to me that the answer is also correct.
- thecopy 3mo ago
  Am i the only one feeling my soul being emptied every time i read another "brutal shorthand" or "honest take"?
  - dbuxton 3mo ago
    It's genuinely depressing.
    - smallmancontrov 3mo ago
      Yeah, but the default is extreme yap mode. I prefer "in a sentence" or "rows, not paragraphs." "Brutal" sounds like bait from an influencer trying to sell me a $400 course about how 4AM workouts will make me a millionaire.
      - jdiff 3mo ago
        I don't even know how we got here. This isn't that deeply represented in the training data. Is this what RLHF hath wrought? A new dialect of English based on corporatespeak and influencers, two heavy-hitting bullshitters?
        deleted 3mo ago
        [deleted]
        ChucklsTheBeard 3mo ago
mlmonkey 3mo ago
"For a human this would not be tricky at all." Which humans have you been hanging out with? :-D I could not make sense of the question at all, and I have a PhD in Computer Science and decades of SWE experience :-D :-D

icepush 3mo ago

Did you ask the question several times in fresh chat contexts to see if it sometimes gives the right answer ?

zythyx 3mo ago
Nah, n=1 is enough to give evidence that something is entirely broken, of course. /s
- 3eb7988a1663 3mo ago
  Well, when we had deterministic tools, it would only take a single example of a calculator claiming 1+1=4 for me to throw it in the trash.
  - IshKebab 3mo ago
    And if you can come up with a deterministic tool that can do everything LLMs can then that would be amazing! Until then, we have to accept the non-determinism.
  - CamperBob2 3mo ago
    That's like saying, "It would only take a single example of a table saw cutting someone's thumb off for me to switch back to hand saws." A noble sentiment, perhaps. But while the table saw user might lose a digit every now and then, you'll get flattened. Determinism is vastly overrated.

3eb7988a1663 3mo ago

I am not a physicist but perhaps your question was leading more than you expected? I would take the question to pre-suppose I have an abundance of the stated material, ignoring practical realities of refinement. If I did have fully pure Pu-241, would that be a better fuel than U-235? Or stated another way, "If you could run a generator on gasoline or jet fuel, which one would you choose and why?" I would answer jet fuel owing to slightly higher energy density and purity of the material - likely leading to a cleaner burn. Which would ignore that jet fuel is going to be a multiple of the gasoline price.

onion2k 3mo ago
If I did have fully pure Pu-241, would that be a better fuel than U-235? Also not a physicist, but I assume from the fact that the OP is asking the LLM this question to trip it up, the point is that U-235 is better even if you have an abundance of both. It's scarcity of Pu-241 leads to the lack of data in training, not that it's actually better.
- 3eb7988a1663 3mo ago
  Again, really speaking out of my depth, but if there is a lack of plutonium training data, I would assume the LLM answer would be the far more commonly described U-235. To respond otherwise means there is some existing association with Pu-241 being better.
  - Skwid 3mo ago
    That's 2.5% more neutrons, surely that must be better!
- adrian_b 3mo ago
  I am lazy to search for a more authoritative source, but Wikipedia says that Pu-241 has a greater neutron absorption cross section than Pu-239 and the same probability of fission after absorbing a neutron. It would also generate a slightly higher amount of energy per fission event, than Pu-239. This means that it would actually be a better fuel than Pu-239 for a fission reactor and only its scarcity prevents its use. The only disadvantage versus Pu-239 is its short half-life, of 14 1/3 years. This means that it cannot be stored for a long time, so it must be consumed as a fuel soon after it is produced, to avoid losses. Pu-239 has a lower delayed neutron fraction than U-235, which makes the control of a Pu-239 fission reactor more difficult. But according to: https://www-nds.iaea.org/sgnucdat/a6.htm https://www-nds.iaea.org/sgnucdat/a6.htm Pu-241 has almost the same delayed neutron fraction as U-235 (0.016 vs. 0.0162), so that is not a serious disadvantage for it. Both Pu-239 and Pu-241 produce more neutrons per fission event than U-235. This simplifies some things, by allowing a reactor to work with less fuel or less-enriched fuel, but it complicates the control, because there is a greater risk of instabilities. The truth is that an LLM cannot say which is a preferable fuel between U-235, Pu-239 and Pu-241. It would be possible to design fission reactors that work fine for any of these 3. The best choice depends on economical factors, not on technical feasibility factors. The only real reason why Pu-241 will never be used is that its production yield when irradiating uranium with neutrons is too low in comparison with Pu-239, so it would be too expensive.

bel8 3mo ago

A more fair and useful comparison would be to feed both LLMs with documentation about such niche knowledge in the contex, then ask.

croes 3mo ago

> For a human this would not be tricky at all. I very much doubt that.

credit_guy 3mo ago
What I meant is: for a human who has the context, like someone who works in the field, or took some courses, or read some materials about this. A human who just has a vague idea about fissile isotopes and needs to google the properties of U-235 or Pu-241 would do no better; they would probably do much worse.

paintbox 3mo ago

For comparison allow me to add chatGPT 5.5: "Choose U-235 if the goal is safe, boring, practical electricity generation. Choose Pu-241 only if the goal is specifically to consume/recycle plutonium in a reactor designed and licensed for that fuel. In brutal shorthand: Pu-241 is a better “fissile isotope” in some nuclear-physics ways, but U-235 is a much better reactor fuel in the real world." If only I knew anything about nuclear reactors. But it sounds to me that the answer is also correct.

thecopy 3mo ago
Am i the only one feeling my soul being emptied every time i read another "brutal shorthand" or "honest take"?
- dbuxton 3mo ago
  It's genuinely depressing.
  - smallmancontrov 3mo ago
    Yeah, but the default is extreme yap mode. I prefer "in a sentence" or "rows, not paragraphs." "Brutal" sounds like bait from an influencer trying to sell me a $400 course about how 4AM workouts will make me a millionaire.
    - jdiff 3mo ago
      I don't even know how we got here. This isn't that deeply represented in the training data. Is this what RLHF hath wrought? A new dialect of English based on corporatespeak and influencers, two heavy-hitting bullshitters?
      - deleted 3mo ago
        [deleted]
      - ChucklsTheBeard 3mo ago

mlmonkey 3mo ago

"For a human this would not be tricky at all." Which humans have you been hanging out with? :-D I could not make sense of the question at all, and I have a PhD in Computer Science and decades of SWE experience :-D :-D